Numerous devices have been developed to test for presence and quantity of analytes in aqueous samples, such as whole blood or urine. The patent and technical literature of the last thirty years is replete with inventions which utilize a reagent strip containing a dry chemistry reagent system, that is, a system in which the wet chemistries are imbibed into an absorbent or bibulous medium, dried, and later reconstituted by fluid from the test sample. The reagent strips contain an indicator which changes color, depending on the presence or concentration of a particular analyte in a biological fluid applied to the strip. These strips may be read visually by reference to a color standard or calorimetrically by an instrument calibrated or programmed to detect a certain color. These strips use reduction chemistries, or an oxidizable dye or dye couple. Some of the strips include an enzyme, such as glucose oxidase, which is capable of oxidizing glucose to gluconic acid and hydrogen peroxide. They also contain an oxidizable dye and a substance having peroxidative activity, which is capable of selectively catalyzing oxidation of the oxidizable dye in the presence of hydrogen peroxide. (See, for example, Phillips et al., U.S. Pat. No. 4,935,346.) Examples of these devices have been described to test for glucose, cholesterol, triglycerides, calcium or albumin in whole blood, and for protein, ketones, albumin or glucose in urine.
Dry chemistry reagent strips incorporating enzyme-based compositions are used daily by millions of diabetics to determine blood glucose concentrations. The NIH sponsored study, the Diabetes Complications and Control Trial, demonstrated conclusively that careful control of blood glucose levels can significantly reduce the incidence of serious complications of diabetes such as vision loss and kidney malfunction. Most diabetics must test themselves periodically in order to make appropriate adjustments to their diet or medication. It is thus especially important for diabetics to have rapid, inexpensive, and accurate test strips for glucose determination.
The technologies embodied in the products which have been developed to date have certain limitations from the perspective of the end user and/or the manufacturer. There is, therefore, a need to overcome some of the limitations of currently available testing systems. Specifically, the embodiment of dry chemistry reagent systems in test strips adds sufficient cost to each test strip as to make them too expensive for some diabetics to use on a daily basis. Technology that eliminates the chemistry added to a test strip would reduce cost to the user and allow simpler manufacturing process for production. A direct measurement system would reduce the annual cost of the measurement dramatically by eliminating much of the reoccurring disposable cost.
Infrared (IR) and Raman spectroscopy are analytical methods that provides qualitative and quantitative information on chemical species such as glucose because the presence and intensity of absorption or emission maxima correlate with the presence and concentration of a functional group within the chemical species. A hand held optical instrument that employs these techniques to measure blood glucose directly without chemistry strips would allow inexpensive testing for people with diabetes.
The major restriction of Raman and IR (including NIR) spectroscopy is the high detection limit of the method which results in the techniques not determining low concentrations of trace chemical species such as glucose. For IR spectroscopy, the key to determining the concentration of a trace chemical species is to increase the intensity of the absorption. This can be done by either a pre-concentration technique (evaporation or transfer into a second matrix) or using a long path length through the sample for the IR beam (long path cuvette, multireflection cuvette, multireflection ATR, optical fibers). The pre-concentration step is time consuming and may change the sample. The longer path length requires a large sample size and will also increase the absorbance of the background. For Raman spectroscopy, the key to determining the concentration of a trace chemical species is to increase the intensity of the emission which may also be accomplished with a pre-concentration technique but has the same drawbacks.
Surface-enhanced infrared absorption (SEIR) and Surface-enhanced Raman (SER) techniques can be used to increase the absorption and emission of the chemical species respectively without the drawbacks stated above. This is because it has been shown that chemical species on or near rough metal surfaces achieve a higher absorption or emission of light. Roughened surfaces of colloidal silver, gold, and copper have all been shown to give increased Raman and IR signals in the presence of trace chemical species such as glucose and the process is reversible when the chemical species is taken away. SEIR has been shown to give IR absorption gains of 50 fold and SER has been shown to enhance the ordinary Raman signal by 1.4 million fold. The ability to enhance the glucose signal by these factors because the sample window has a layer of colloidal metal will allow practical detection of solution glucose at physiological concentrations without added chemistry on the test device.
Since the signal enhancement is restricted to the chemical species near the vicinity of the surface, the interface can be selectively monitored without interference from the solution background. There are many different forms of substrate suitable for surface enhancement of IR or Raman signal; these forms include, for example, colloids, electrodes, coated microspheres, fumed silica and acid etched metal surfaces.
Phillips et al., U.S. Pat. No. 4,935,346 describes a system wherein a whole blood sample is applied to the device and indicator development occurs in the presence of the colored components of the sample. Measurements of the color change in indicator are made at two distinct wavelengths to eliminate the interferences from the presence of colored blood components.
Muller, U.S. Pat. No. 4,427,889, describes an apparatus for infrared spectroscopy using two different wavelengths to effect a quantitative measurement in biological medium specifically for the determination of products of metabolism. The level of blood glucose can be determined through absorption analysis of infrared wavelengths absorbed in a glucose containing samples. This approach requires high concentrations of the target component due to the low signal of the technique and the dual wavelength nature of the measurement.
Kaiser, U.S. Pat. No. 4,169,676 describes a method of determining the content of metabolic products in blood using a laser beam that is guided through an attenuated total reflectance (ATR) plate which is placed directly against the skin. Knudson, U.S. Pat. No. 5,079,421; Knudson, U.S. Pat. No. 5,179,951 and Braig, WO 95/31930 describe infrared spectrometric noninvasive methods for measuring the concentration of blood glucose. The methods detect the absorption of infrared radiation by glucose in human tissues.
Braig, U.S. Pat. No. 5,313,941; Clift, WO 91/18548 and Clift, WO 93/09421 describe techniques of applying infrared radiation having multiple wavelengths to measure glucose in blood without interference from overlapping components. Short bursts or pulses of radiation are employed to prevent burning of the tissue.
Berger et al., U.S. Pat. No. 5,615,673, describes an apparatus for measuring analytes in blood and tissue using Raman spectroscopy. The method is suitable for in vitro and in vivo transdermal blood analysis.
Alsmeyer et al., U.S. Pat. No. 5,610,836, describes a method of applying radiation having multiple wavelengths to a sample containing an unknown constituent. Analysis of a sample of unknown constitution often produces data that are perturbed by conditions prevailing at the data collection site. By analyzing multiple variables in a matrix format, a calibration function to determine constituent concentration can be obtained that adjusts for operational variability associated with the measuring apparatus and other changes in the measurement volume.
Despite the advances in conventional devices and methods, the art is still in search of improved techniques for analysis of biological fluid samples, and especially for analysis of glucose in blood. In particular, there is a need for a portable, inexpensive, and easy to use device for glucose detection that does not require chemical reagents.